As mentioned elsewhere, OSTree is strongly influenced by package
manager designs as well. This page is not intended to be an
exhaustive list of such projects, but we will try to keep it up to
date, and relatively agnostic.

Broadly speaking, projects in this area fall into two camps; either
a tool to snapshot systems on the client side (dpkg/rpm + BTRFS/LVM),
or a tool to compose on a server and replicate (ChromiumOS, Clear
Linux). OSTree is flexible enough to do both.

Note that this section of the documentation is almost entirely
focused on the "ostree for host" model; the flatpak
project uses libostree to store application data, distinct from the
host system management model.

Combining dpkg/rpm + (BTRFS/LVM)

In this approach, one uses a block/filesystem snapshot tool underneath
the system package manager.

Regarding BTRFS
in particular - the OSTree author believes that Linux storage is a
wide world, and while BTRFS is quite good, it is not everywhere now,
nor will it be in the near future. There are other recently developed
filesystems like f2fs, and Red
Hat Enterprise Linux still defaults to
XFS.

Using a snapshot tool underneath a package manager does help
significantly. In the rest of this text, we will use "BTRFS" as a
mostly generic tool for filesystem snapshots.

The obvious thing to do is layer BTRFS under dpkg/rpm, and have a
separate subvolume for /home so rollbacks don't lose your data. See
e.g. Fedora BTRFS Rollback Feature.

More generally, if you want to use BTRFS to roll back changes made by
dpkg/rpm, you have to carefully set up the partition layout so that
the files laid out by dpkg/rpm are installed in a subvolume to
snapshot.

This problem in many ways is addressed by the changes OSTree forces,
such as putting all local state in /var (e.g. /usr/local ->
/var/usrlocal). Then one can BTRFS snapshot /usr. This gets pretty
far, except handling /etc is messy. This is something OSTree does
well.

In general, if one really tries to flesh out the BTRFS approach, a
nontrivial middle layer of code between dpkg/rpm and BTRFS (or deep
awareness of BTRFS in dpkg/rpm itself) will be required. A good
example of this is the snapper.io project.

The OSTree author believes that having total freedom at the block
storage layer is better for general purpose operating systems. For
example, the ability to choose dm-crypt per deployment is quite useful;
not every site wants to pay the performance penalty. One can choose
LVM or not, etc.

Where applicable, OSTree does take advantage of copy-on-write/reflink
features offered by the kernel for /etc. It uses the now generic
ioctl(FICLONE) and copy_file_range().

Another major distinction between the default OSTree usage and package managers
is whether updates are "online" or "offline" by default. The default OSTree
design writes updates into a new root, leaving the running system unchanged.
This means preparing updates is completely non-disruptive and safe - if the
system runs out of disk space in the middle, it's easy to recover. However,
there is work in the rpm-ostree
project to support online updates as well.

OSTree supports using "bare-user" repositories, which do not require
root to use. Using a filesystem-level layer without root is more
difficult and would likely require a setuid helper or privileged service.

Finally, see the next portion around ChromiumOS for why a hybrid but
integrated package/image system improves on this.

ChromiumOS updater

Many people who look at OSTree are most interested in using
it as an updater for embedded or fixed-purpose systems, similar to use cases
from the ChromiumOS updater.

The ChromiumOS approach uses two partitions that are swapped via the
bootloader. It has a very network-efficient update protocol, using a
custom binary delta scheme between filesystem snapshots.

This model even allows for switching filesystem types in an update.

A major downside of this approach is that the OS size is doubled on
disk always. In contrast, OSTree uses plain Unix hardlinks, which
means it essentially only requires disk space proportional to the
changed files, plus some small fixed overhead.

This means with OSTree, one can easily have more than two trees
(deployments). Another example is that the system OSTree repository
could also be used for application containers.

Finally, the author of OSTree believes that what one really wants for
many cases is image replication with the ability to layer on some
additional components (e.g. packages) - a hybrid model. This is what
rpm-ostree is aiming
to support.

Clear Linux Software update

Like OSTree static deltas, it also uses bsdiff for network efficiency.

More information will be filled in here over time. The OSTree author
believes that at the moment, the "CL updater" is not truly atomic in
the sense that because it applies updates live, there is a window
where the OS root may be inconsistent.

casync

The systemd casync project is
relatively new. Currently, it is more of a storage library, and doesn't
support higher level logic for things like GPG signatures, versioning
information, etc. This is mostly the OstreeRepo layer. Moving up to
the OstreeSysroot level - things like managing the bootloader
configuration, and most importantly implementing correct merging for /etc
are missing. casync also is unaware of SELinux.

OSTree is really today a shared library, and has been for quite some time.
This has made it easy to build higher level projects such as
rpm-ostree which has quite
a bit more, such as a DBus API and other projects consume that, such as
Cockpit.

A major issue with casync today is that it doesn't support garbage collection
on the server side. OSTree's GC works symmetrically on the server and client
side.

Broadly speaking, casync is a twist on the dual partition approach, and
shares the general purpose disadvantages of those.

Mender.io

OLPC update

OSTree is basically a generalization of olpc-update, except using
plain HTTP instead of rsync. OSTree has the notion of separate trees
that one can track independently or parallel install, while still
sharing storage via the hardlinked repository, whereas olpc-update
uses version numbers for a single OS.

OSTree has built-in plain old HTTP replication which can be served
from a static webserver, whereas olpc-update uses rsync (more server
load, but more efficient on the network side). The OSTree solution to
improving network bandwidth consumption is via static deltas.

NixOS / Nix

See NixOS. It was a very influential project for OSTree.
NixOS and OSTree both support the idea of independent "roots" that are bootable.

In NixOS, files in a package are accessed by a path depending on the checksums
of package inputs (build dependencies) - see
Nix store.
However, OSTree uses a commit/deploy model - it isn't tied to any particular
directory layout, and you can put whatever data you want inside an OSTree, for
example the standard FHS layout. A both positive and negative of the Nix model
is that a change in the build dependencies (e.g. being built with a newer gcc),
requires a cascading rebuild of everything. It's good because it makes it easy
to do massive system-wide changes such as gcc upgrades, and allows installing
multiple versions of packages at once. However, a security update to e.g. glibc
forces a rebuild of everything from scratch, and so Nix is not practical at
scale. OSTree supports using a build system that just rebuilds individual
components (packages) as they change, without forcing a rebuild of their
dependencies.

Nix automatically detects runtime package dependencies by scanning content for
hashes. OSTree only supports only system-level images, and doesn't do dependency
management. Nix can store arbitrary files, using nix-store --add, but, more
commonly, paths are added as the result of running a derivation file generated
using the Nix language. OSTree is build-system agnostic; filesystem trees are
committed using a simple C API, and this is the only way to commit files.

OSTree automatically shares the storage of identical data using hard links into
a content-addressed store. Nix can deduplicate using hard links as well, using
the auto-optimise-store option, but this is not on by default, and Nix does not
guarantee that all of its files are in the content-addressed store. OSTree
provides a git-like command line interface for browsing the content-addressed
store, while Nix does not have this functionality.

Nix used to use the immutable bit to prevent modifications to /nix/store, but
now it uses a read-only bind mount. The bind mount can be privately remounted,
allowing per-process privileged write access. OSTree uses the immutable
bit on the root of the deployment, and mounts /usr as read-only.

NixOS supports switching OS images on-the-fly, by maintaining both booted-system
and current-system roots. It is not clear how well this approach works. OSTree
currently requries a reboot to switch images.

Finally, NixOS supports installing user-specific packages from trusted
repositories without requiring root, using a trusted daemon.
Flatpak, based on OSTree, similarly has a
policykit-based system helper that allows you to authenticate via polkit to
install into the system repository.

Solaris IPS

See
Solaris IPS. Broadly,
this is a similar design as to a combination of BTRFS+RPM/deb. There
is a bootloader management system which combines with the snapshots.
It's relatively well thought through - however, it is a client-side
system assembly. If one wants to image servers and replicate
reliably, that'd be a different system.

Conary

See
Conary Updates and Rollbacks. If
rpm/dpkg are like CVS, Conary is closer to Subversion. It's not bad,
but e.g. its rollback model is rather ad-hoc and not atomic. It also
is a fully client side system and doesn't have an image-like
replication with deltas.

bmap

See
bmap.
A tool for optimized copying of disk images. Intended for offline use,
so not directly comparable.

Git

Although OSTree has been called "Git for Binaries", and the two share the idea
of a hashed content store, the implementation details are quite different.
OSTree supports extended attributes and uses SHA256 instead of Git's SHA1. It
"checks out" files via hardlinks, rather than copying, and thus requires the
checkout to be immutable. At the moment, OSTree commits may have at most one
parent, as opposed to Git which allows an arbitrary number. Git uses a
smart-delta protocol for updates, while OSTree uses 1 HTTP request per changed
file, or can generate static deltas.

Conda

Conda is an "OS-agnostic, system-level binary
package manager and ecosystem"; although most well-known for its accompanying
Python distribution anaconda, its scope has been expanding quickly. The package
format is very similar to well-known ones such as RPM. However, unlike typical
RPMs, the packages are built to be relocatable. Also, the package manager runs
natively on Windows. Conda's main advantage is its ability to install
collections of packages into "environments" by unpacking them all to the same
directory. Conda reduces duplication across environments using hardlinks,
similar to OSTree's sharing between deployments (although Conda uses package /
file path instead of file hash). Overall, it is quite similar to rpm-ostree in
functionality and scope.

rpm-ostree

This builds on top of ostree to support building RPMs into OSTree images, and
even composing RPMs on-the-fly using an overlay filesystem. It is being
developed by Fedora, Red Hat, and CentOS as part of Project Atomic.

GNOME Continuous

This is a service that incrementally rebuilds and tests GNOME on every commit.
The need to make and distribute snapshots for this system was the original
inspiration for ostree.

Docker

It makes sense to compare OSTree and Docker as far as wire formats
go. OSTree is not itself a container tool, but can be used as a
transport/storage format for container tools.

Docker has (at the time of this writing) two format versions (v1 and
v2). v1 is deprecated, so we'll look at format version 2.

A Docker image is a series of layers, and a layer is essentially JSON
metadata plus a tarball. The tarballs capture changes between layers,
including handling deleting files in higher layers.

Because the payload format is just tar, Docker hence captures
(numeric) uid/gid and xattrs.

This "layering" model is an interesting and powerful part of Docker,
allowing different images to reference a shared base. OSTree doesn't
implement this natively, but it's not difficult to implement in higher
level tools. For example in
flatpak, there's a concept of a
SDK and runtime, and it would make a lot of sense for the SDK to
depend on the runtime, to avoid clients downloading data twice (even
if it's deduplicated on disk).

That gets to an advantage of OSTree over Docker; OSTree checksums
individual files (not tarballs), and uses this for deduplication.
Docker (natively) only shares storage via layering.

The biggest feature OSTree has over Docker though is support for
(static) deltas, and even without pre-configured static deltas, the
archive format has "natural" deltas. Particularly for a "base
operating system", one really wants on-wire deltas. It'd likely be
possible to extend Docker with this concept.

A core challenge both share is around metadata (particularly signing)
and search/discovery (the ostree summary file doesn't scale very
well).

One major issue Docker has is that it checksums compressed data,
and furthermore the tar format is flexible, with multiple ways to represent data,
making it hard to impossible to reassemble and verify from on-disk state.
The tarsum effort
was intended to address this, but it was not adopted in the end for v2.

Docker-related: Balena

The Balena project forks Docker and aims
to even use Docker/OCI format for the root filesystem, and adds wire deltas
using librsync. See also discussion on libostree-list.